Changes enabling the elimination of processes for a torsion bar

ABSTRACT

In one aspect of the invention, a torsion bar assembly is provided. The assembly includes a first shaft having a first bore, a second shaft having a second bore, the second shaft operatively coupled to the first shaft, and a torsion bar positioned within the first and second bores. The torsion bar includes a splined first end having a first diameter extending to a first end face having a diameter generally the same as the first diameter, a splined second end having a second diameter extending to a second end face having a diameter generally the same as the second diameter, and an active diameter extending between the splined first end and the splined second end. The torsion bar is fabricated from a material having a hardness greater than 45 Rockwell C-Scale.

FIELD OF THE INVENTION

The subject invention relates to a torsion bar, and more particularly,to a torsion bar for a power steering assembly.

BACKGROUND OF THE INVENTION

A power steering assembly of a vehicle may include a power assisteddevice that facilitates the turning of a steering wheel by a vehicleoperator. In order to achieve the function of the power steering, it maybe necessary to provide a torsion bar. However, processes required tofabricate the torsion bar to a specified rate may be expensive and timeconsuming, particularly if it includes a profile grind cycle process.Further, it was historically believed that torsion bars needed to befabricated from relatively soft materials with little or no hardening ofthe material because the assembly process typically involved drillingand reaming, which reduced tool life. Thus, torsion bars were nothardened in order to increase tool life. A secondary function of thetorsion bar is to regain the original neutral position of the steeringwheel after the steering wheel is turned and then relieved of torque;commonly known as hysteresis. This led to long torsion bars thatrequired multiple processing steps during manufacture. Accordingly, itis desirable to provide a torsion bar with shorter length, lowerhysteresis, and that undergoes less manufacturing processes.

SUMMARY OF THE INVENTION

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

In one aspect of the invention, a torsion bar for a steering columnassembly is provided. The torsion bar includes a splined first endhaving a first diameter extending to a first end face having a diametergenerally the same as the first diameter, a splined second end having asecond diameter extending to a second end face having a diametergenerally the same as the second diameter, and an active diameterextending between the splined first end and the splined second end.

In another aspect of the invention, a torsion bar assembly for asteering column assembly is provided. The assembly includes a firstshaft having a first bore, a second shaft having a second bore, thesecond shaft operatively coupled to the first shaft, and a torsion barpositioned within the first and second bores. The torsion bar includes asplined first end having a first diameter extending to a first end facehaving a diameter generally the same as the first diameter, a splinedsecond end having a second diameter extending to a second end facehaving a diameter generally the same as the second diameter, and anactive diameter extending between the splined first end and the splinedsecond end.

In yet another aspect of the invention, a method of fabricating atorsion bar having a first end and a second end is provided. The methodincludes performing a shear operation on the torsion bar, performing aserration rolling operation on the first and second ends, and performinga hardening process on the torsion bar, the hardening process hardeningthe torsion bar to a hardness of between 48 and 55 Rockwell C-Scale.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of a prior art torsion bar;

FIG. 1A is a schematic illustration of a torsion bar according to afirst embodiment;

FIG. 1B is a schematic illustration of a torsion bar according to asecond embodiment;

FIG. 2 is a cross-sectional view of a prior art torsion bar assembly;

FIG. 2A is a cross-sectional view of a torsion bar assembly according toa first embodiment;

FIG. 2B is a cross-sectional view of a torsion bar assembly according toa second embodiment;

FIG. 3 is a cross-sectional view of a shaft of the prior art assemblyshown in FIG. 2;

FIG. 3A is a cross-sectional view of a shaft of the exemplary assembliesshown in FIGS. 2A and 2B;

FIG. 4 is a schematic illustration of exemplary manufacturing processesaccording to the invention;

FIG. 5 is a chart of design enhancements of one of the exemplary torsionbars shown in FIG. 1;

FIG. 6 is a enlarged side view of a portion of one of the exemplarytorsion bars shown in FIG. 1; and

FIG. 7 is a chart of design enhancement of one of the exemplary torsionbars shown in FIG. 1.

DETAILED DESCRIPTION

Described herein are systems and methods for manufacturing areduced-length torsion bar with lower hysteresis and less processingsteps than previously known torsion bars. The torsion bars describedherein undergo increased hardening, which facilitates production of ashorter torsion bar that does not require processing steps such aswashing, profile grinding, or shot peening, for example.

Referring now to the Figures, where the invention will be described withreference to specific embodiments, without limiting same, illustrated isa prior art torsion bar 10 (FIG. 1) for a steering column assembly, anexemplary torsion bar 100 (FIG. 1A) for a steering column assemblyaccording to the invention, and another exemplary torsion bar 200 (FIG.1B) for a steering column assembly according to the invention. FIG. 2illustrates a prior art torsion bar assembly 12, FIG. 2A illustrates anexemplary torsion bar assembly 102 according to the invention, and FIG.2B illustrates another exemplary torsion bar assembly 202 according tothe invention.

As shown in FIG. 1, prior art torsion bar 10 includes a bullet nose 14,a press chamfer 16, a splined large diameter 18, a blend 20, and anactive diameter 22. As shown in FIGS. 2 and 3, torsion bar assembly 12includes a first shaft 24, a second shaft 26, and prior art torsion bar10. Second shaft 26 includes a drilled clearance bore 28 to maintainclearance to active diameter 22, a press bore 30 to engage splined largediameter 18, and a pre-drill bore 32.

With reference to FIGS. 1-3, torsion bar 10 is pressed into bore 28 andincludes bullet nose 14 to seat on a drill tip cone 34 of bore 28.Historically, press loads in the pressing operation were kept as low aspossible. Typically, press interference and therefore tolerances of thespline and press bore diameters were kept small. The tight tolerancesforced the need for a reaming operation to create bore 30 in shafts 24,26 and a grinding operation on the tips of splines 18 of torsion bar 10.Before reaming could be done, a pre-drill operation was performed toremove most of the material because precision reaming depends on only aslight material removal. Thus, after drilling bore 28, a pre-drilloperation was required to create bore 32 from drill cone 30 to the endlocation of clearance bore 28. Then a reamer was used to form a pressbore 36 and produce tighter tolerances. However, a shoulder 38 (FIG. 3)created by the reamer was not big enough to act as a final stop for theaxial pressing operation of torsion bar 10 due to variation of drillsizes, reamer sizes, and bore true positioning. This could producesituations in which torsion bar 10 had large variations in its bottomload and axial press location. In order to alleviate issues that couldarise with shoulder 38, bullet nose 14 was added to prior art torsionbar 10 to produce a definitive measurable bottom load variation bycontacting drill cone 30. However, bullet nose 14 added length totorsion bar 10, which required extra drilling depth for drill bore 28.

Press chamfer 16 was added to prior art torsion bar 10 to both minimizepress loads and reduce incidents of galling. Active diameter 22 hashistorically been designed at the upper limit of the allowable torsionalstress required for fatigue life and hysteresis. Large diameter feature18 provides sufficient material by increasing to a diameter larger thanactive diameter 22 in order to transfer torque from torsion bar 10 tomating shaft 24, 26. Without this extra material, the combination oftorsional stress and the stress from the interface between torsion bar10 and shafts 24, 26 would reduce fatigue life and add hysteresis.

Blend 20 was added to torsion bar 10 to prevent adding a stress riser asthe diameter increased from active diameter 22 to large diameter 18. Aprofile grind process is used in the manufacture of torsion bar 10 dueto the tight tolerances required for active diameter 22 or torsionalrate and at the interface large diameter 18 to reduce pressinterference. Bullet nose 14, press chamfer 16, and blend 20 areadditionally created during the profile grind to reduce the number ofprocess steps. However, the profile grind process may be time consumingand expensive. Further, the long torsion bar 10 requires more material,which may be costly, and shafts 24 and/or 26 may require time consumingand more costly drilling, reaming, and/or pinning steps to configure tothe longer torsion bar.

As illustrated in FIGS. 1A and 2A, torsion bar 100 is a cylindrical typetorsion bar having a circular cross-section and includes a splined firstend 108, a splined second end 110, a large diameter 118, a blend 120,and an active diameter 122. Splined first end 108 extends to a first endface 109, and first end face 109 has a diameter generally the same asthe diameter of splined first end 108. As used herein, the term“generally the same” means the same diameter or a diametrical differencethat occurs natural or is only expediently formed to facilitateassembly, as further described herein. Splined second end 110 extends toa second end face 111, and second end face 111 has a diameter generallythe same as the diameter of splined second end 110. Active diameter 122has a diameter that is less than the large diameter 118 of splined firstand second ends 108, 110.

Torsion bar 100 has a length ‘L1’ that is less than a length ‘L’ ofprior art torsion bar 10. For example, in one embodiment, length ‘L’ isapproximately 126 mm and length ‘L1’ is between 90 mm and 110 mm orbetween approximately 90 mm and approximately 110 mm. In anotherembodiment, ‘L1’ is between 95 mm and 105 mm or between approximately 95mm and approximately 105 mm. In yet another embodiment, ‘L1’ is between100 mm and 104 mm or between approximately 100 mm and approximately 104mm.

As shown in FIG. 2A, torsion bar assembly 102 includes a first shaft124, a second shaft 126, and a drill bore 128. However, drill bore 128has a depth ‘D1’ that is less than a depth ‘D’ of prior art drill bore28 (FIG. 3) and does not include a reamer bore 30 or shoulder 38. Thus,torsion bar 100 requires less material and a shorter drilling time forbore 128 than prior art torsion bar 10. In one embodiment, depth ‘D1’ isbetween 20 mm and 30 mm shorter than depth ‘D’ or between approximately20 mm and approximately 30 mm shorter. In another embodiment, depth ‘D1’is between approximately 25 mm and 27 mm shorter or betweenapproximately 25 mm and 27 mm shorter than depth ‘D’.

In the exemplary embodiment, torsion bar 100 undergoes a hardening stepto increase the hardness of torsion bar 100 such that it is harder thanprior art torsion bar 10. This enables torsion bar 100 to bemanufactured with length ‘L1’ that is less than prior art length ‘L’ andreduces hysteresis (e.g., by approximately 50%). Although prior arttorsion bar 10 may be subjected to a slight hardening process, it isonly done for reduction of galling during the press operation, and bar10 is only hardened to an upper limit of 40 Rockwell C-Scale hardness.Thus, prior art torsion bar 10 is still considered a “soft” torsion bar.

In contrast, torsion bar 100 is subjected to a hardening process and ishardened to greater than or equal to 45 Rockwell C-Scale hardness. Inone embodiment, torsion bar 100 is hardened to between 48 and 55Rockwell C-Scale hardness or between approximately 48 and approximately55 Rockwell C-Scale hardness. Torsion bar 100 has a higher hardness thantorsion bar 10, which enables the reduced length ‘L1’ while providingreduced hysteresis. Further, increased hardness allows more pressinterference, which allows more size tolerance to be given to both pressbores 128 and the spline major diameter.

Due to the increased hardness of torsion bar 100, it does not have toundergo a shot peening process like prior art torsion bar 10. Theprocess of shot peening is a process required in soft torsion bar 10 inorder to increase fatigue life in torsion bar 10. Thus, a shot peeningstep is not required for torsion bar 100 because the higher hardnesssignificantly increases the yield point, and the reduction in themaximum twist angle reduces the stress. In other words, torsional stressis a function of torque applied, diameter, and twist angle, and reducingthe twist angle reduces the stress.

As previously discussed, bullet nose 14 was included in torsion bar 10in order to reach around shoulder 38 created at the interface betweenthe pre-drill and the reamer features. This was to ensure that the bar10 bottomed out on drill tip cone 34 to make the press versusdisplacement curves easier to read and predict. However, with torsionbar 100, elimination of the reaming operation allows the elimination ofa bullet nose, which increases the life of the grinding wheel (notshown), due to the fact that this is the deepest ground portion of theshaft, and further reduces cost.

FIG. 4 illustrates a manufacturing process 104 for torsion bar 100 thatincludes a shear step 140, a center-less grinding step 142, a serrationrolling step 144, a wash step 146, a hardening step 148, a wash step150, a profile grind step 152, and a wash and oil step 154. Shear step140 may include cutting the torsion bar blank to the correct length,center-less grinding step 142 may include feeding through a grinder, andserration rolling step 144 may include forming splines into torsion bar100. In wash step 146, torsion bar may be flooded with lubricant andwashed, in hardening step 148 torsion bar 100 may be placed in a furnaceand heated (e.g., to about 1500°) and subsequently quenched, and in washstep 150 torsion bar 100 may be washed to remove quench oil. In wash andoil step 154, torsion bar 100 may be washed from contaminants (e.g.,iron powder) and given an anti-rust coating of oil.

As illustrated, compared to prior art manufacturing processes, process104 eliminates a shot peen process 156 due to the increased hardness oftorsion bar 100, as discussed above.

As shown in FIG. 5, the design changes or enhancements of torsion bar100 compared to prior art torsion bar 10 provide the followingattributes, which are illustrated in chart 106. In the exemplaryembodiment, (1) elimination of the bullet nose 160 facilitateselimination of the ream operation 162; (2) an increase in the torsionbar hardness 164 facilitates an increase in torsion bar yield stress166, which facilitates elimination of the shot peen process 168, anincrease in maximum allowable press interference 170 (along with anincreased press machine load capacity 172), and a reduced torsion barlength 174. The increased maximum allowable press interference 170facilitates an increase in spline outer diameter tolerance 176, and anincreased press bore tolerance 178, which facilitates elimination of theream operation 162; (3) a reduced maximum angular twist 184 decreasestorsion bar torsional stress 186 and facilitates reduced torsion barlength 174; and (4) reduced torsion bar length 174 facilitates reduceddrilling depth 188 in shafts 124, 126. Thus, as illustrated, designenhancements 160, 164, 174, 178, and 184 facilitate manufacturingimprovements 162, 168, 176, and 188.

As illustrated in FIGS. 1B and 2B, torsion bar 200 is a cylindrical typetorsion bar having a circular cross-section and includes first andsecond splined ends 208 and 210. Splined first end 208 extends to afirst end face 209, and first end face 109 has a diameter generally thesame as the diameter of splined first end 208. As used herein, the term“generally the same” means the same diameter or a diametrical differencethat occurs natural or is only expediently formed to facilitateassembly. Splined second end 210 extends to a second end face 211, andsecond end face 211 has a diameter generally the same as the diameter ofsplined second end 210.

Torsion bar 200 has a length ‘L2’ that is less than length ‘L’ of priorart torsion bar 10 For example, in one embodiment, length ‘L’ isapproximately 126 mm and length ‘L2’ is between 90 mm and 110 mm orbetween approximately 90 mm and approximately 110 mm. In anotherembodiment, ‘L2’ is between 95 mm and 105 mm or between approximately 95mm and approximately 105 mm. In yet another embodiment, ‘L2’ is between100 mm and 104 mm or between approximately 100 mm and approximately 104mm.

As shown in FIG. 2B, torsion bar assembly 202 includes a first shaft224, a second shaft 226, and a drill bore 228. However, drill bore 228has a depth ‘D2’ that is less than depth ‘D’ of prior art drill bore 28(FIG. 3) and does not include a reamer bore 30 or shoulder 38. Moreover,torsion bar 200 does not require a bullet nose, press chamfer, largediameter, blend, or profile grind step. Thus, torsion bar 100 requiresless material and a shorter drilling time of bore 228 than prior arttorsion bar 10. In one embodiment, depth ‘D2’ is between 20 mm and 30 mmshorter than depth ‘D’ or between approximately 20 mm and approximately30 mm shorter. In another embodiment, depth ‘D2’ is betweenapproximately 25 mm and 27 mm shorter or between approximately 25 mm and27 mm shorter.

In the exemplary embodiment, torsion bar 200 undergoes a hardening stepto increase the hardness of torsion bar 200 such that it is harder thanprior art torsion bar 10. This enables torsion bar 20 to be manufacturedwith length ‘L2’ that is less than prior art length ‘L’ and reduceshysteresis (e.g., by approximately 50%). Although prior art torsion bar10 may be subjected to a slight hardening process, it is only done forreduction of galling during the press operation, and bar 10 is onlyhardened to an upper limit of 40 Rockwell C-Scale hardness. Thus, priorart torsion bar 10 is still considered a “soft” torsion bar.

In contrast, torsion bar 200 is subjected to a hardening process and ishardened to greater than or equal to 45 Rockwell C-Scale hardness. Inone embodiment, torsion bar 200 is hardened to between 48 and 55Rockwell C-Scale hardness or between approximately 48 and approximately55 Rockwell C-Scale hardness. Torsion bar 200 has a higher hardness thantorsion bar 10, which enables the reduced length ‘L2’ while providingreduced hysteresis. Further, increased hardness allows more pressinterference, which allows more size tolerance to be given to both pressbores 228 and the spline major diameter

Due to the increased hardness of torsion bar 200, it does not have toundergo a shot peening process like prior art torsion bar 10. Theprocess of shot peening is a process required in soft torsion bar 10 fora drill/ream/pin system, for example, in order to increase fatigue lifein torsion bar 10. Thus, a shot peening step is not required for torsionbar 200 because the higher hardness significantly increases the yieldpoint, and the reduction in the maximum twist angle reduces the stress.

As previously discussed, bullet nose 14 was included in torsion bar 10in order to reach around shoulder 38 created at the interface betweenthe pre-drill and the reamer features. This was to ensure that the bar10 bottomed out on drill tip cone 34 to make the press versusdisplacement curves easier to read and predict. However, with torsionbar 200, elimination of the reaming operation allows the elimination ofa bullet nose, which increases the life of the grinding wheel (notshown) and further reduces cost.

Large diameter 18 was included in torsion bar 10 to provide enoughmaterial to transfer torque without distorting the torsion bar, whichcould lead to reduced life and hysteresis. However, the hardness levelof torsion bar 10 is limited because drilling into hardened materialsreduces tool life and necessitates frequent changing of the drills. Incontrast, torsion bar 200 no longer requires the large diameter shape,and the manufacturing process after adding the serrations is eliminated.

Moreover, the pre-roll diameter of splined ends 208, 210 is equal to orsubstantially equal to an active diameter 222 (FIG. 1-1B). As such,after the serration rolling process, the minor diameter 215 of theserrations is smaller than the diameter of active diameter 222.

In addition, torsion bar 200 has a smaller spline diameter and fewerteeth than prior art torsion bar 10, which is accomplished by reducingthe maximum twist angle allowed in torsion bar 200. In order to achievethis, a new torsion bar centering machine is provided that holds abetter tolerance on the centering process. This reduction in centeringsubsequently enables the maximum twist angle to be reduced.

In the exemplary embodiment, a press chamfer 16 is eliminated fortorsion bar 200. However, as illustrated in FIG. 6, a natural blend 234occurs because the end of torsion bar 200 is unrestrained. Thus, duringa serration rolling process, the metal follows the path of leastresistance causing axial growth at the outer edge 236 of torsion bar200. This causes a non-fill condition in the die which may then be usedas a lead-in chamfer/blend for the pressing operation. It should benoted that natural blend 234 occurs naturally, while devoted processesare used to form press chamfer 16 and blend 20 in torsion bar 10.

FIG. 4 illustrates a manufacturing process 204 for torsion bar 200 thatincludes a shear step 240, a center-less grinding step 242, a serrationrolling step 244, a wash step 246, a hardening step 248, and a wash andoil step 254. As illustrated, compared to prior art manufacturingprocesses, process 204 eliminates a wash step 250, a profile grind step252, and a shot peen process 256 due to the increased hardness oftorsion bar 100, as discussed above.

As shown in FIG. 7, the design changes or enhancements of torsion bar200 compared to prior art torsion bar 10 provides the followingattributes, which are illustrated in chart 206. In the exemplaryembodiment, (1) elimination of the bullet nose 260 facilitateselimination of the ream operation 262; (2) an increase in the torsionbar hardness 264 facilitates an increase in torsion bar yield stress266, which facilitates elimination of the shot peen process 268, anincrease in maximum allowable press interference 270 (along with anincreased press machine load capacity 272), and a reduced torsion barlength 274. The increased maximum allowable press interference 270facilitates an increase in spline outer diameter tolerance 276, and anincreased press bore tolerance 278, which facilitates elimination of theream operation 262; (3) a reduced maximum angular twist 284 decreasestorsion bar torsional stress 286. Decreased torsional stress 286facilitates reduced torsion bar length 274 and eliminates a largediameter or any diameter (larger than the active diameter) shape 290,which facilitates elimination of a profile grind step 292. The increasespline outer diameter tolerance 276 also facilitates elimination of theprofile grind step 192; (4) reduced torsion bar length 274 facilitates areduced drilling depth 288 in shafts 224, 226; and (5) elimination ofthe profile grind 292 facilitates production of pointed spline tips 294,which facilitates deformation of the press bore material from side toside 296 (not radially). This facilitates increasing the maximumallowable press interference 270. Thus, as illustrated, designenhancements 260, 264, 274, 276, 278, 284, and 290 facilitatemanufacturing improvements 262, 268, 288, and 292.

Described herein are systems and methods for improved torsion barmanufacture. By making design enhancements in the torsion barmanufacture, previously required processes may be reduced or eliminated.For example, the torsion bar may undergo a hardening process, whichenables the torsion bar to be shorter than previous designs. As such, abullet nose, large diameter, and chamfers that were once required fortorsion bars may now be eliminated. Further, processing steps such asshot peening, reaming, and profile grinding may be eliminated.Accordingly, the improved torsion bars described herein provide the samefunction as previous torsion bars, but with reduced cost and processingtime.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description.

Having thus described the invention, it is claimed:
 1. A torsion bar for a steering column assembly, the torsion bar comprising: a splined first end having a first diameter extending to a first end face having a diameter generally the same as the first diameter; a splined second end having a second diameter extending to a second end face having a diameter generally the same as the second diameter; and an active diameter extending between the splined first end and the splined second end; and a minor diameter of the splined first end, the minor diameter being less than the active diameter.
 2. The torsion bar of claim 1, wherein at least one of the splined first and second ends does not include a bullet nose.
 3. The torsion bar of claim 1, wherein at least one of the first and second diameters is generally equal to the active diameter.
 4. The torsion bar of claim 3, wherein the at least one of the first and second splined ends does not include a blend between the large diameter and the active diameter.
 5. The torsion bar of claim 1, wherein the torsion bar has a length of between 90 mm and 110 mm.
 6. The torsion bar of claim 1, wherein the torsion bar is fabricated from a material having a hardness of between 48 and 55 Rockwell C-Scale.
 7. A torsion bar assembly for a steering column assembly, the torsion bar assembly comprising: a first shaft having a first bore; a second shaft having a second bore, the second shaft operatively coupled to the first shaft; and a torsion bar positioned within the first and second bores, the torsion bar comprising: a splined first end having a first diameter extending to a first end face having a diameter generally the same as the first diameter; a splined second end having a second diameter extending to a second end face having a diameter generally the same as the second diameter; and an active diameter extending between the splined first end and the splined second end.
 8. The torsion bar assembly of claim 7, wherein at least one of the first and second diameters is equal to the active diameter.
 9. The torsion bar assembly of claim 8, wherein the at least one of the splined first and second ends does not include a blend between the large diameter and the active diameter.
 10. The torsion bar assembly of claim 7, wherein at least one of the splined first and second ends does not include a bullet nose.
 11. The torsion bar assembly of claim 7, wherein the torsion bar has a length between 90 mm and 110 mm.
 12. The torsion bar assembly of claim 7, wherein the torsion bar is fabricated from a material having a hardness of between 4$ and 55 Rockwell C-Scale.
 13. The torsion bar assembly of claim 7, wherein the first bore does not include a reamer bore.
 14. The torsion bar assembly of claim 7, wherein a minor diameter of the splined first end is less than a diameter of the active diameter.
 15. A method of fabricating a torsion bar having a first end and a second end, the method comprising: performing a shear operation on the torsion bar; performing a serration rolling operation on the first and second ends to form a splined first end and a splined second end, the minor diameter of the first splined end being less than an active diameter extending between the splined first end and the splined second end; performing a hardening process on the torsion bar, the hardening process hardening the torsion bar to a hardness of between 48 and 55 Rockwell C-Scale.
 16. The method of claim 15, wherein a shot peen process is not performed on the torsion bar.
 17. The method of claim 15, wherein a profile grinding process is not performed on the torsion bar.
 18. The method of claim 15, wherein the torsion bar is formed with a length between 90 mm and 110 mm. 